Endangered Species Recovery Program

giant kangaroo ratDipodomys ingens

Status

U.S.A. and California Endangered

Life History

Giant kangaroo rats are curious and bold inhabitants of the most arid,
southwestern edge of central California's San Joaquin Valley and adjacent
valleys and plateaus of the Inner Coastal ranges. They are found from
elevations of about 90 to 885 meters, though today, most extant habitat
lies at elevations above 200 meters. Few colonies are located above 760
meters. Giant kangaroo rats mainly inhabit sandy-loam soils located on
level and gently sloping ground vegetated with annual grasses and forbs
and widely-scattered desert shrubs. Extant habitat has been fragmented,
mostly by irrigated croplands, into six major geographic units, which
in turn have been broken into dozens of smaller colonies by agricultural
and petroleum developments. Fossil giant kangaroo rats are known from
late Pleistocene (38,000 years before present) brea deposits in an area
still inhabited.

Though they often emerge from their burrows in the twilight
hours around sunset, and less frequently in the daytime, giant
kangaroo rats are mainly active at night. Except when harvesting
seeds at the end of the growing season of herbaceous plants, they
are out of their burrows for only about 15 minutes per night
during a 1.8-hour period starting near sunset.

Giant kangaroo rats are inveterate diggers, frequently
remodeling their burrows, closing old entrances and creating new
ones. They usually live solitary lives within their shallow
burrows, which average less than 30 centimeters in depth and have
branching side tunnels that typically do not reconnect. Besides
an enlarged chamber for nesting, which may or may not contain
nesting material such as plant fibers and animal hairs, there are
several enlarged chambers where seeds are stored. There may be up
to 24 of these larders in an individual's burrow, some empty or containing
seed hulls and others each containing 1-9 liters of seeds, mostly
peppergrass (Lepidium spp.), filaree (Erodium
cicutarium), Arabian grass (Schismus arabicus), and
brome grasses (Bromus spp.). As herbaceous plants ripen,
giant kangaroo rats cut the seed head and place them either in
shallow, thimble-shaped pits arranged in a honey-comb pattern and
covered with dirt after filling, or in densely compacted piles on
the surface. Surface piles are sometimes enormous for such a
small animal. In one study, a stack was found that measured 1.2
by 1.8 meters and was 10 centimeters deep; in another, stacks
averaged 29 liters, and ranged from about 2.5 to 53.2 liters.
Seedheads in stacks cure in the sun for about 4-6 weeks before
being moved to larders in the burrows. Besides seeds, giant
kangaroo rats eat small amounts of green foliage such as leaves
of clover (Trifolium depauperatum) and filaree, and
insects.

Long-term occupancy of a site by giant kangaroo rats results
in a mima-mound topography, with burrow systems located on mounds
a few to several centimeters above the intervening ground. The
mound is the center of the individual's territory, which is tiny
in comparison to most kangaroo rats and other small mammals,
averaging about 0.04 ha. These precincts of individual kangaroo
rats support lusher, greener vegetation richer in nitrogen than herbaceous
plants on surrounding ground. Productivity of plants on precincts
averages about 3-5 times greater, and plant species composition
on precincts differs from vegetation on surrounding ground,
consisting mostly of plants with larger seeds favored by the
kangaroo rats. Following harvest of the seed heads and dying of
the annual vegetation, the kangaroo rat clears the plant litter
from its precinct, creating a barren contrast with surrounding, vegetated
ground.

The owner of a precinct with its granaries vigorously
defends it against all intruders. A giant kangaroo rat produces
long, rapid drumming sounds by alternately striking its huge hind
feet on the ground while standing only on its hind feet.
Presumably, this is one aspect of their territoriality. Drumming
in the burrows and on the surface is greatest in late spring and
summer during and after seed harvest. During the day, interloping
birds and antelope squirrels (Ammospermophilus nelsoni),
bent on stealing from the surface caches, are aggressively chased
from the precinct. The giant kangaroo rat emerges from its burrow
through a vertical shaft, and, with its boldly black-and-white-striped
tail held stiffly curved upward, runs at top speed at the
intruder. When it reaches the boundary of its territory, it
reverses, and at full speed dives down the shaft, its long,
upward-projecting tail being the last to disappear.

The better habitats for giant kangaroo rats are shared with
few or no other small, nocturnal mammals. San Joaquin antelope
squirrels are the only other common mammal. During the day they
enter the openings of giant kangaroo rat burrows, usually
reappearing shortly. Presumably they are using only shallow
portions of tunnels not plugged at the surface to shelter from
heat and predators. Occasionally, the occupant of a burrow
emerges and chases off the antelope squirrel.

Giant kangaroo rats have major impacts on their communities,
increasing and enriching plant productivity, being the base of
the food chain for most predatory vertebrates, providing
sheltering burrows for the endangered blunt-nosed leopard lizard
(Gambelia sila), threatened antelope squirrel, and other
animals, and providing favorable microhabitats for the endangered
plants, San Joaquin woolly-threads (Lembertia congdoni
and California jewelflower(Caulanthus californicus). Where
shrubs such as saltbushes (Atriplex spp.) and snakeweed
(Gutierrezia spp.) are recruited during rare, heavy,
late-spring rains, giant kangaroo rats gnaw through the stems,
eventually eradicating most or all shrubs on their precincts and
surrounding ground.

Most females enter estrus in the cool, wet winter in central
California, in mid or late December or January. When population
density is high and most precincts are occupied, adult females
may have only a single litter of from 1-4 young after a gestation
period of about 32 days. Under these circumstances,
young-of-the-year do not breed. During years of drought and low
or no seed production, females are monestrous or anestrous.
During years with a prolonged wet season or where population
density is low and there are many vacant precincts, adult females
may have 2-3 litters and young-of-the-year females may begin
breeding when about 12-13 weeks old. Young giant kangaroo rats appear
on the surface when they weigh about 50-70 or more grams and are
presumably about 6.5 to 8.5 weeks old. Severe drought results in
population decline, mainly by reduction or cessation of
reproduction; whereas torrential rainfall over several days
results in great, rapid population decline, presumably by
drowning, death from wetting and hypothermia, and other factors
related to too much moisture.

Size

Identification

Of the species of kangaroo rats sharing its geographic range, D.
ingens is distinguished from Dipodomys heermanni by
longer hind foot, wider head, and greater mass. D. nitratoides
is much smaller in all characters and has four instead of five
toes on the hind foot.